The present invention relates to a magneto-optical recording medium such as a magneto-optical disk, magneto-optical tape, a magneto-optical card, etc., for use in a magneto-optical recording and reproducing device.
One type of re-writable optical memory medium conventionally implemented for actual use is a magneto-optical disk using a magneto-optical memory medium. In such a magneto-optical disk, a light beam projected by a semiconductor laser is condensed and projected onto the magneto-optical recording medium, thereby raising the temperature of a localized area of the magneto-optical recording medium to perform recording and erasure. Accordingly, recorded information is reproduced by condensing and projecting onto the magneto-optical recording medium a light beam of a strength insufficient to perform recording and erasure, and distinguishing a state of polarization of light reflected therefrom.
However, in this type of magneto-optical recording medium, as the diameter of recorded bits of recorded magnetic domains and the interval between recorded bits becomes small in comparison with the diameter of the light beam spot, reproducing characteristics deteriorate. This is because adjacent recording bits fall within the light beam spot condensed on a target recording bit, thus making it impossible to distinguish and reproduce the individual recorded bits.
One magneto-optical recording medium which has been proposed to solve the foregoing problem is a magneto-optical recording medium structured of a reproducing layer having in-plane magnetization at room temperature and shifting to perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer having a Curie temperature in the vicinity of the critical temperature, a non-magnetic intermediate layer, and a recording layer made of a perpendicular magnetized film, for recording information (disclosed in Japanese Unexamined Patent Publication No. 9-320134/1997 (Tokukaihei 9-320134), published on Dec. 12, 1997).
In the foregoing conventional magneto-optical recording medium, the reproducing layer has in-plane magnetization at temperatures below the critical temperature. Accordingly, at temperatures below the critical temperature, the in-plane magnetization of the reproducing layer forms a mask, so that recorded magnetic domain information recorded in the recording layer is not copied to the reproducing layer, and thus the recorded magnetic domain information is not reproduced. At temperatures at and above the critical temperature, however, the reproducing layer shifts to perpendicular magnetization. Accordingly, at temperatures at and above the critical temperature, recorded magnetic domain information is copied to the reproducing layer, and recorded magnetic domain information is reproduced.
With the foregoing structure, even if adjacent recorded bits fall within the light beam spot condensed on the reproducing layer, the individual recorded bits can be distinguished and reproduced, as long as reproducing power of the light beam and the critical temperature at which the reproducing layer shifts to perpendicular magnetization are set appropriately. Consequently, it is possible to reproduce information recorded at high density, i.e., to perform ultra-high resolution reproducing.
The following will explain, with reference to FIG. 13, an ultra-high resolution magneto-optical recording medium, which is a magneto-optical recording medium capable of reproducing information recorded at high density. FIG. 13 is an explanatory drawing showing the principle of ultra-high resolution reproducing operations in a conventional magneto-optical recording medium.
The foregoing conventional ultra-high resolution magneto-optical recording medium is structured of a reproducing layer 101 having in-plane magnetization at room temperature and perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer 102 having a Curie temperature in the vicinity of the critical temperature, a non-magnetic intermediate layer 103, and a recording layer 104 made of a perpendicular magnetized film having a compensation temperature in the vicinity of room temperature.
Reproducing is performed by condensing and projecting a light beam 105 onto the reproducing layer 101 side of the ultra-high resolution magneto-optical recording medium. Condensing and projecting the light beam 105 onto the ultra-high resolution magneto-optical recording medium forms therein a temperature distribution having a Gaussian distribution corresponding to an intensity distribution of the light beam 105. In accordance with this temperature distribution, the reproducing layer 101 shifts from in-plane to perpendicular magnetization, forming a domain 106 having a temperature at or above the critical temperature and having perpendicular magnetization. Within domains of the reproducing layer 101 retaining in-plane magnetization, an in-plane magnetization mask is formed, and thus a reproducing signal is not produced.
In the domain 106 where the reproducing layer 101 has shifted to perpendicular magnetization, on the other hand, total magnetization is directed in the same direction (up or down) as the direction of magnetic flux leaking from the recording layer 104. Accordingly, the direction of magnetization of the recording layer 104 is copied to the reproducing layer 101, and ultra-high resolution reproducing can be realized.
Here, the in-plane magnetized layer 102, whose Curie temperature is in the vicinity of the critical temperature, is exchange-coupled with the reproducing layer 101, and is provided in order to strengthen the in-plane magnetization mask in domains of the reproducing layer 101 whose temperature is less than the critical temperature.
As discussed above, in the foregoing conventional ultra-high resolution magneto-optical recording medium, it is preferable to reproduce only the information of the domain 106 having perpendicular magnetization, where the reproducing layer 101 has a temperature at or above the critical temperature.
Here, in order to give the reproducing layer 101 characteristics whereby it has in-plane magnetization at room temperature and shifts to perpendicular magnetization with rising temperature, the composition of the reproducing layer 101, in contrast to a compensation composition in which moment of rare earth (RE) and transition metal (TM) sub-lattices are the same size in a temperature range at which reproducing is performed, must be an RE-rich composition having more RE sub-lattice moment. In the reproducing layer 101, the orientation of the TM sub-lattice moment and that of total magnetization are parallel, but are directed in opposite directions, i.e., they are anti-parallel.
The recording layer 104, on the other hand, is made of an RE-TM alloy having a compensation temperature at room temperature, and in the temperature range at which reproducing is performed, the TM sub-lattice moment is larger than the RE sub-lattice moment. Accordingly, in the recording layer 104, the orientation of the TM sub-lattice moment and that of total magnetization are parallel, and are directed in the same direction (up or down).
However, in the foregoing conventional ultra-high resolution magneto-optical recording medium, the total magnetization of the in-plane magnetized layer 102 gradually decreases as temperature rises, and in the vicinity of the domain 106, it is difficult for the in-plane magnetized layer 102 to strengthen the in-plane magnetization mask of the reproducing layer 101. Consequently, in the vicinity of the domain 106, even at temperatures below the critical temperature, the reproducing layer 101 is influenced by leaking magnetic flux produced by the recording layer 104, and the orientation of the magnetization of the reproducing layer 101 tilts with respect to the surface of the layer. Accordingly, when reproducing information from the domain 106, information is simultaneously reproduced from adjacent domains, even though they are below the critical temperature, and this impairs reproducing resolution.
Given that in recent years there is a need for even larger recording capacity, the foregoing magneto-optical recording medium disclosed in Japanese Unexamined Patent Publication No. 9-320134/1997, as discussed above, provides insufficient in-plane magnetization masking effect, and thus cannot provide sufficient reproducing resolution.
It is an object of the present invention to improve reproducing resolution of a magneto-optical recording medium so as to enable reproducing of individual recorded bits with high signal quality, even when information is recorded at high density.
In order to attain the foregoing object, a magneto-optical recording medium according to the present invention comprises a reproducing layer having in-plane magnetization at room temperature and shifting to have perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer made of an in-plane magnetized film, and a recording layer made of a perpendicularly magnetized film, provided in that order, wherein the in-plane magnetized layer includes a first in-plane magnetized layer having a Curie temperature in the vicinity of the critical temperature, and a second in-plane magnetized layer having a Curie temperature which is higher than the Curie temperature of the first in-plane magnetized layer; with the first in-plane magnetized layer being provided toward the reproducing layer, and the second in-plane magnetized layer being provided toward the recording layer.
With the foregoing structure, the in-plane magnetized layer provided between the reproducing layer and the recording layer includes a first in-plane magnetized layer and a second in-plane magnetized layer. The first in-plane magnetized layer is provided toward the reproducing layer, and has a Curie temperature in the vicinity of the critical temperature of the reproducing layer. The second in-plane magnetized layer, on the other hand, is provided toward the recording layer, and has a Curie temperature higher than that of the first in-plane magnetized layer.
At temperatures below the Curie temperature of the first in-plane magnetized layer, the first and second in-plane magnetized layers are exchange coupled with each other. Consequently, the reproducing layer is exchange coupled with the second in-plane magnetized layer via the first in-plane magnetized layer.
Here, as the first in-plane magnetized layer is heated to its Curie temperature in the vicinity of the critical temperature of the reproducing layer, its magnetization decreases, and thus the force holding the reproducing layer in its in-plane magnetized state weakens. However, at this time, the second in-plane magnetized layer, which has a Curie temperature higher than the Curie temperature of the first in-plane magnetized layer, i.e., higher than the critical temperature of the reproducing layer, has sufficiently large magnetization. Accordingly, since the reproducing layer is, via the first in-plane magnetized layer, exchange coupled with the second in-plane magnetized layer having a sufficiently large magnetization, the magnetization of the reproducing layer is firmly fixed in an in-plane state, even in domains having temperatures close to the critical temperature.
When, on the other hand, the temperature of the first in-plane magnetized layer exceeds its Curie temperature, the reproducing layer is no longer exchange coupled to the second in-plane magnetization layer, and the reproducing layer shifts to perpendicular magnetization, and the direction of magnetization (up or down) of the recording layer is copied to the reproducing layer and reproduced.
In this way, stronger in-plane magnetization forms a mask in the reproducing layer, and leakage of magnetic flux from the recording layer can be completely shut out, even in domains of the reproducing layer in the vicinity of the domain with perpendicular magnetization, which have temperatures close to the critical temperature.
Consequently, even with a small recorded bit diameter and a small recorded bit interval, it is possible to obtain a reproducing signal with sufficiently high resolution, i.e., to improve reproducing resolution in magnetic ultra-high resolution reproducing.
Further, a magneto-optical recording medium according to the present invention may be structured so as to comprise a reproducing layer having in-plane magnetization at room temperature and shifting to have perpendicular magnetization at temperatures at and above a critical temperature, an in-plane magnetized layer made of an in-plane magnetized film, and a recording layer made of a perpendicularly magnetized film, provided in that order, wherein Curie temperature of the in-plane magnetized layer changes continuously in the direction of layer thickness, such that Curie temperature at an interface with the recording layer is higher than Curie temperature at an interface with the reproducing layer.
With the foregoing structure, Curie temperature of the in-plane magnetized layer between the reproducing and recording layers changes continuously in the direction of thickness of the layer, so that Curie temperature at the interface with the recording layer is higher than Curie temperature at the interface with the reproducing layer, and in domains whose temperature is less than the Curie temperature, the in-plane magnetized layer is exchange coupled to the reproducing layer.
Accordingly, during reproducing, as that part of the in-plane magnetized layer adjacent to the reproducing layer is heated to its Curie temperature, magnetization decreases, and the force holding the reproducing layer to an in-plane magnetized state weakens. However, since, as mentioned above, the in-plane magnetized layer has a Curie temperature distribution in the direction of layer thickness, with increasing distance from the reproducing layer and increasing proximity to the recording layer, Curie temperature increases, and magnetization of the in-plane magnetized layer is larger. For this reason, the reproducing layer is exchange coupled to that part of the in-plane magnetized layer provided farthest from the reproducing layer (on the recording layer side).
As that part of the in-plane magnetized layer adjacent to the reproducing layer is heated above its Curie temperature, the reproducing layer is no longer exchange coupled to that part of the in-plane magnetized layer, and the reproducing layer shifts to perpendicular magnetization, and the direction of magnetization (up or down) of the recording layer is copied to the reproducing layer and reproduced.
In this way, stronger in-plane magnetization forms a mask in the reproducing layer, and leakage of magnetic flux from the recording layer can be completely shut out, even in domains of the reproducing layer in the vicinity of the domain with perpendicular magnetization, which have temperatures close to the critical temperature.
Consequently, even with a small recorded bit diameter and a small recorded bit interval, it is possible to obtain a reproducing signal with sufficiently high resolution, i.e., to improve reproducing resolution in magnetic ultra-high resolution reproducing.
Further, it is preferable if the foregoing magneto-optical recording medium is further provided with a nonmagnetic intermediate layer between the in-plane magnetized layer and the recording layer.
With this structure, exchange coupling between the in-plane magnetized layer and the recording layer is shut out by the non-magnetic intermediate layer.
If the non-magnetic intermediate layer is not provided, the in-plane magnetization of the in-plane magnetized layer is subject to influence by exchange coupling acting between the in-plane magnetized layer and the recording layer, and it is necessary to make the in-plane magnetized layer thicker to avoid this influence. For this reason, by shutting out exchange coupling between the in-plane magnetized layer and the recording layer by providing, as in the present invention, a non-magnetic intermediate layer made of a non-magnetic material, the in-plane magnetized layer can be made thinner.
Consequently, decreased thickness of the in-plane magnetized layer makes it possible to improve recording sensitivity.
Further, it is preferable if the foregoing magneto-optical recording medium is also provided with a reflective layer between the non-magnetic intermediate layer and the recording layer, and if the non-magnetic intermediate layer is transparent.
With this structure, by providing the reflective layer between the non-magnetic intermediate layer and the recording layer, an interference effect produced in the multi-layered structure results in increased Kerr rotation angle. Further, the reproducing light beam which has passed through the reproducing layer, the in-plane magnetized layer, and the transparent non-magnetic intermediate layer is reflected by the reflective layer.
Accordingly, reproducing of unnecessary signals from the recording layer can be more effectively prevented.
Incidentally, Kerr rotation angle is an angle of rotation of a plane of polarization of light, which rotates according to the direction of magnetization of a magnetic body when the light is reflected from a surface of or passes through the magnetic body.
Consequently, it is possible to reproduce only the information copied to the reproducing layer, thus improving both ultra-high resolution reproducing characteristics and reproducing signal quality.